Gustard R26 finally excels via I2S input!

Same with Denafrips. I run Terminator II via Hermes DDC and I2S. I had same impression as you on standalone Gustard.

My setup is more modest than many here, but my Gustard x26 pro sounds better in the I2S input using a VERY inexpensive Douk Audio DDC with a good LPS and an inexpensive HDMI Cable

I run a Denafrips Pontus II with a Hermes DDC via I2S.  Before the addition of the DDC I thought the DAC was excellent but the after the DDC took it to another level.

My journey is an extension of yours. I started using I2s to connect the Jay's cdt2-mk3 to Gustard X26 PRO. Used a Tubulus Concentus cable, which I thought couldn't be beat. Then I came across the very interesting concept of the Snake River aes cable and to my surprise the Jay's sounded just as revealing as with the Concentus but with a more solid presentation which is very much in line with my choice of music, mainly rock and electronic. That opened the door for using the Gustard X26 PRO I2s connection for something else. I had been using the Innuos ZENith mk3 connected to the X26 pro via Network acoustics eno III usb cable. The result was excellent but it had a tendency to accentuate the low frequencies more so than the Jay's and I always had to adjust my subs to compensate between the two. The sound was good either way but I had a slight preference for the Jay's which sounded a little more airy, yet plentiful in the low end. Of course to use the Innuos via I2s I had to get a DDC. Got the U18. And bingo, the sound of the streamer took it at least to the level of the Jay's, even with a slightly larger presentation. In fairness, I use the C18 master clock to clock the LHY SW-10 ethernet switch, the U18 DDC and the X26 pro DAC in chain gang fashion via square outputs. I now use strictly the NOS setting as I no longer feel like I benefit from oversampling at all. That's how clear everything sounds. And I'm now done. But I have said that many times before lol. It's a journey allright. Big part of the fun. However, the house of stereo is what makes it all possible for me. With every update I make, I know that it will sound as good as it possibly can. And if it doesn't sound better, I sell it without thinking twice about it.